Cliff swallows evolve to speed past automobiles

Nick Farrell, 19th March 2013

Cliff swallows are evolving so fast that they have developed shorter wings to deal with the threat of speeding cars.

Scientists looking at the numbers of cliff swallows killed along roads in south western Nebraska have notices that the numbers have become much less and the birds’ average wing length has shrunk.

According to the latest issue of Current Biology, the data shows that evolution can be a lot faster than most people think.

Animal behaviorist Colleen Cassady St. Clair of the University of Alberta in Edmonton said that the data was “jaw dropping.” The results suggest that years of smacking into cars forced swallows to adapt to the road.

For millions of years, cliff swallows built nests under overhangs on cliff faces. But when highway bridges were built they thought these would make a nice modern home for the family.

Study author Charles Brown of the University of Tulsa in Oklahoma said the birds twigged that their nests were less likely than cliff nests to be washed away in storms. The only downside is the fast moving traffic.

Initially Brown and study co-author Mary Bomberger Brown wanted to investigate the bird’s social behaviour in the new colonies. Every summer for the next 29 years, the team trekked to the colonies, counted nests and picked up dead birds. In total, the Browns gathered more than 2,000 swallows.

But from 1983, the researchers collected fewer birds killed by cars each year, until they found only four in 2012. Initially they though that it was something to do with the fact that birds were no longer trying to read the number plates of cars or some sort of social conditioning, but then they realised that the bird’s wing lengths were getting shorter. Swallows that died on the road had wings that were a few millimetres longer.

This few millimetres was enough to let the birds take off quickly and manoeuvre through the air. The swallows evolved short wings that help them to take off quicker.

Other potential explanations, such as declining swallow populations or an increase in avian scavengers stealing carcasses had been ruled out. Brown says factors other than wing length may also be involved. Cars may have killed off daredevil swallows, for example, leaving more cautious birds behind. But that does not explain why the wings have become shorter.